1. Technical Field
The embodiments relate to an image sensor, and more particularly to a pixel cell having a plurality of epitaxial layers, a CMOS image sensor including the pixel cell, and a method of forming a pixel cell of the CMOS image sensor.
2. Description of the Related Art
A complementary metal oxide semiconductor (CMOS) image sensor is widely used as an image capturing device because it has low operating voltage, low power consumption, and low production cost. The CMOS image sensor includes a pixel cell array composed of a plurality of pixel cells. Each of the pixel cells may include a photo diode, a transfer transistor, a reset transistor, a source follower transistor and a row selecting transistor. The photo diode includes a charge accumulating portion that accumulates charge generated by a light.
A thick epitaxial layer is required to decrease loss in a silicon area of electrons generated by a light incident on the photo diode. However, when the thickness of the epitaxial layer is increased, a cross-talk between neighboring photo diodes may be increased. Therefore, the effective depth of the photo diode needs to be increased to decrease the cross-talk between the photo diodes.
Conventionally, ion implantation process has been performed using simply a high energy, and thus a phenomenon called blooming may be caused between photo diodes when a distance between the neighboring photo diodes is relatively short. The blooming is a phenomenon where electrons overflow between the photo diodes. A photo diode may be formed in a stepped epitaxial layer or in a graded epitaxial layer using a count-doping method without additional photo processes.
However, in the pixel cell of an image sensor having a stepped epitaxial layer or a graded epitaxial layer, there is a limit to the degree to which an effective depth of the photo diode region can be increased by raising energy of an ion beam in the ion implantation process.